1. Field of Invention
The goal of the present invention is to supply heavy-metal-free glass enamels that can be applied to glass and fired. After firing, the coated glass can subsequently be ion exchanged to give a chemically strengthened, decorated glass article. One possible application of such a process would be the decoration of thin glass for electronic devices.
2. Description of Related Art
Glass enamel compositions are well known in the art. One aim of conventional glass and enamel compositions is the achievement of a low firing, high durability glass and enamel having a low coefficient of thermal expansion (CTE).
Chemically strengthened, substrate glass is known in the art. For example, Gorilla Glass®, manufactured by Corning, is made by exchanging the Na ions near the surface of the glass for K+ ions in an ion exchange bath. The larger K+ ions create chemical pressure at the surface of the glass, helping to strengthen the glass. Heating the glass above approximately 400° C. causes the potassium to migrate deeper into the glass, diminishing the effect. For that reason, decorating the ion-exchanged glass with fired-on enamels is not possible. Other types of commercially available chemically strengthened glasses are Schott Xensation®, AGC Dragontrail®, and NEG CX-01.
Glass enamels have been used for many years to decorate automotive, building and appliance glass. The glasses used in these enamels must have softening points well below those of the substrate glasses. For that reason, small alkali metals such as lithium and sodium are sometimes used in relatively large amounts in the glasses to reduce their softening points. Such glasses should theoretically be good candidates for ion exchange, provided that the softening point of the glass used in the enamel is also high enough to avoid dissolution in the ion exchange bath. The enamel should also be chemically stable toward the ion exchange bath.
Glass enamels with lower coefficients of expansion (CTE) than the substrate glass have been designed. These materials can be coated onto glass and fired to fuse them to the substrate. Upon cooling, the glass article is strengthened on the surface by the compression created by the enamel which contracts less.